The objectives of this research are to determine the mechanisms of thyroxine biosynthesis and H2O2 production in the thyroid gland. These objectives will be pursued as follows: 1) The coupling of diiodotyrosyl residues to form thyroxine and triiodothyronine during the iodination of proteins or peptides is an oxidative reaction. The principal investigator has recently discovered that "active iodine" (either I plus or IO minus) generated by various peroxidases (including the thyroid enzyme) oxidatively cleaves tryptophan and diiodotyrosine peptides. The relationship between the oxidative fission of DIT peptides and DIT coupling will be investigated by iodinating polypeptides and proteins. Synthesis of T4 and T3 will be specifically measured by competitive protein binding and radioimmunoassay, and the cleavage of peptide bonds will be measured by the appearance of new N-terminal residues with the sensitive fluorescamine reaction. 2) The rate of H2O2 production in the rat thyroid gland will be determined by a specific and sensitive procedure utilizing cytochrome c peroxidase. The effect of TSH (stimulant) and various goitrogens (inhibitors) in vivo on H2O2 production will be established to gain more insight on the mode of action of these agents. The rate of H2O2 production in glands from patients with Pendred's syndrome will be assessed and compared to normal human thyroid glands to determine whether there is a deficiency in H2O2 production in this disease.